The invention is directed to an electrographic printer comprising an intermediate image carrier for the simultaneous printing of a first web section of an endless carrier material and of a second web section arranged next to the first web section at a distance therefrom. A transport device conducts the web sections along a transport path past the intermediate image carrier. At least one tension device pre-stresses the first web section and the second web section against the intermediate image carrier during the printing process. At least one web tenser is arranged following the transport device as viewed in the transport direction and which gives the web sections a tensile stress in the transport direction during the printing process. A compensation device is arranged between the transport device and the tension device that pre-stresses the first web section or the first and second web sections opposite the transport direction during the printing process.
The invention is concerned with an improvement of an electrographic printer unit for printing band-shaped recording media of different band width according to WO 94/27193. The printer unit disclosed therein has an electrographically working intermediate image carrier, for example a photoconductor drum, a usable width of which corresponds to twice the width format of a standard form according to DIN A4 or of a letter size format. The further units, such as the fixing station, the developer station, the cleaning station, etc., are likewise designed for this usable width.
Various operating modes are possible with this known printer unit. In what is referred to as the simplex mode, thus a recording medium having up to twice the width of a sheet in the DIN A4 format or in the letter size format can be printed in a traditional way. In a parallel simplex mode, two narrow recording media arranged side-by-side, for example with a width according to DIN A4, can be conducted through the printer unit and printed in juxtaposed position.
In a further operating mode, the single-color duplex mode, the web of the recording medium is turned over upon transport through the printer unit. During the first printing event, a first web section of the web is conducted past with the front side at the transfer printing location and is subsequently supplied to the fixing station in which the toner image is joined in smear-proof fashion to the continuous carrier material under the influence of heat and pressure. After the fixing, the web is laterally offset, turned over and resupplied to the transfer printing station in the form of a second web section and is printed on the back side. In this way, it is possible to simultaneously print the first and second web section at the same transfer printing location. A two-color duplex mode is also possible by employing differently colored ink particles in different developer units of the printer unit.
In another operating mode, the two-color simplex mode, the web is offset parallel by at least one web width upon transport in the printer unit, and the offset web sections are conducted past the transfer printing location in common in juxtaposed position. During the first pass of the web at the transfer printing location, image and text elements are printed with a first color and are subsequently fixed; image and text elements are printed with the second color during the second pass of the web with offset.
The printer unit disclosed by WO 94/27193 works according to the principle of electrophotography, whereby a photoconductor on which a latent charge image corresponding to the print format to be printed is applied with the assistance of a light source, for example a laser of an LED line, is employed as a photoconductor. Ink particles of a desired color are transferred onto the photoconductor within a developer station arranged close to the photoconductor and the charge image is inked with toner particles. The developer station is followed by the print unit in which a corona device arranged at a distance from the photoconductor transfers the toner image onto the continuous carrier material with an electrical field, the continuous carrier material being passed through between photoconductor and corona device and being subsequently supplied to the fixing station in which the toner image is joined in smear-proof fashion to the continuous carrier material by heat and pressure. During the transfer of the toner image, the air between the corona device and the continuous carrier material is ionized by high field strengths, as a result of which charge carriers are generated on that side of the continuous carrier material facing away from the photoconductor. An electrostatic force field between continuous carrier material and photoconductor arises as a result thereof, this effecting, first, an adhesion of the continuous carrier material to the surface of the photoconductor and, second, a transfer of the toner image located on the photoconductor onto the continuous carrier material. Since the speed with which the surface of the photoconductor moves is slightly higher than the speed with which the continuous carrier material is conducted past the photoconductor, the continuous carrier material adhering to the surface of the photoconductor due to the force of attraction is entrained and does not separate from the surface of the photoconductor until it is outside the transfer printing station. This undefined release of the continuous carrier material from the surface of the photoconductor occurs irregularly, as a result of which smeared print formats and positional errors of the print format that deteriorate the quality of the print format arise.
For this reason, the known printer provides a compensation unit between the transport device and the tension device, this compensation unit extending over approximately the entire width of the photoconductor, pressing uniformly against that side of the first and/or second web section facing away from the photoconductor and pre-stressing the first and/or second web section with equal force opposite the transport direction, as a result of which a counter-force opposing the force of attraction is produced that leads to a defined release of the web section or, sections from the surface of the photoconductor in the region of the transfer printing location.
During the duplex mode and the two-color simplex mode, a shrinkage occurs during the fixing process of the first web section, particularly due to the influence of heat, i.e. a shortening occurs of the continuous carrier material that is resupplied to the transfer printing location as the second web section. Due to the shortening of the continuous carrier material, the compensation device is moved away from the photoconductor due to the shortened second web section that has already run through the fixing station. As a result thereof, the unshortened first web section is no longer pre-stressed by the compensation device, so that it separates from the surface of the photoconductor in undefined fashion outside the transfer printing location, which results in smeared print formats and positional errors of the print format that deteriorate the quality of the print image.